TLP structures were originally developed for the offshore oil or gas industry. Examples thereof are described in the international patent application WO2010082153 and U.S. Pat. No. 6,910,438.
TLP structures are now currently being considered also in wind turbine applications, particularly in offshore wind turbine applications.
In general, TLP structures for offshore wind turbines comprise a platform and a buoyant or floating structure designed to be placed on the sea. In cases where the platform is a closed structure to be placed under the sea level, the platform itself may act as a buoyant structure. A TLP structure may further comprise a transition piece that joins the wind turbine tower and the platform together.
The buoyant structure and the platform are held in place by means of anchoring legs or tendons acting as mooring lines that are fastened to the seabed. The buoyant structure and the platform are designed to be stable enough to support the tower and the nacelle assembly of the wind turbine in varying weather conditions.
Document WO2009131826 discloses, for example, an offshore wind turbine platform to be anchored to the seabed through mooring lines. An asymmetric mooring system connects the seabed to the column that carries the wind turbine.
In offshore wind turbine applications, the tendons in the TLP structures are formed of high strength tubular members such as cables. The tendons are provided with articulated connections to fix their ends to the corners of the platform and to the seabed. The tendons may also be made of steel concrete as shown in document US2008014025.
The high axial stiffness of the tendons allows the horizontal movement of the platform (surge and sway motion), and at the same time it does not allow the vertical movement (heave motion) and the rotational movement (pitch and roll motions) of the platform caused by the wind turbine operation, the wind and the waves.
Wind turbines are currently producing increasingly more power and consequently they are requiring increasingly larger sizes. In addition, both the tension leg platforms and the offshore wind turbines must be manufactured taking into consideration the elastic behaviour of the Eigen frequencies of the pitch and roll movement in the wind turbine and the platform structure caused by the wind turbine operation, the wind and the waves. In addition, the frequencies of heave movement must be also kept below given values.
The goal is therefore preventing the frequency of the assembly from being the same as that of the waves. The frequencies of heave, surge and sway modes are very low due to low modal associated stiffness and high modal inertia that causes the assembly to act as a rigid structure. The fact that these frequencies are lower than those of the waves results in that the structure dynamics is not excited by the waves.
By contrast, the frequencies of pitch, roll and yaw movements are higher but sometimes they may decrease to become close to those of the waves. Therefore, it is desired that the frequencies of pitch, roll and yaw movements are higher. This is typically achieved by oversizing the tendons, for example, increasing their wall thickness or increasing the number of cables per tendon, for an increased structural strength and stiffness in order to withstand the involved forces. This however undesirably results in increased costs.
A TLP structure is proposed herein intended to provide enhanced mechanical properties in terms of strength and stiffness while reducing costs.